An operational problem that may occur in a heat pump type pool heater (utilizing either a dedicated “heat only” or reversible heating/cooling heat pump circuit) is the generation of high hydraulic pressure spikes occurring upon start-up of the heater after it has been idle (i.e., in the “off” mode) for several days. A sudden increase (“spike”) in hydraulic pressure tends to exceed the setting of the over-pressure sensor of the heater which results in the automatic termination of the operation of the unit's compressor. If this pressure spike-created shutoff occurs three times, the control system locks out the compressor which requires the owner to manually reset the control or call a service technician to place the unit back in operation.
It is believed that this pressure spike problem is caused by too much refrigerant migrating to the relatively small volume water-to-refrigerant heat exchanger used on heat pump pool heaters that fills up the space inside the heat exchanger during the off cycle over an extended time. When the compressor starts, there is not enough gaseous refrigerant between the compressor and expansion device to absorb the sudden increase in pressure produced by the scroll compressor on unit start-up.
This issue is most prevalent on larger size heat pump pool heaters with large evaporator coils and high refrigerant charges relative to the water heater exchanger. Standard receivers help somewhat, but are not that effective and require the addition of costly refrigerant. Historically, the main way to eliminate the nuisance tripping of the pressure sensor is to simply reduce the refrigerant charge of the unit, thereby undesirably lowering the unit's water heating capacity.
As can be seen from the foregoing, a need exists for apparatus that eliminates or at least substantially diminishes this compressor start-up pressure spike problem in a heat pump pool heater or other type of heat pump-based liquid heater. It is to this need that the present invention is primarily directed.